Mortality of marine turtles bycaught in industrial fisheries operating off North-Western Africa

Abstract

We use data collected by onboard observers to assess the bycatch and immediate mortality of sea turtles in the Canary Current Large Marine Ecosystem (CCLME) and adjoining areas from 2016 to 2021. First, a dataset including 698 sets by drifting longliners, 6300 tows by trawlers and 1029 sets by purse seiners was used to calculate the catch per unit of effort for each species and fleet. Second, effort data compiled by the International Commission for the Conservation of Atlantic Tunas (ICCAT) and Global Fishing Watch were used to escalate bycatch to the whole fleet. Third, we used post-release mortality rates from the literature to estimate the total number of turtles killed annually and provide a baseline to assess their potential demographic impact. Accordingly, the international fleet of longliners was estimated to cause the annual mortality of 6800 loggerhead turtles Caretta caretta (95% CI: 5825–7628), 740 olive ridley turtles Lepidochelys olivacea (95% CI: 560–926) and smaller numbers of hawksbill Eretmochelys imbricate and green turtles Chelonia mydas. Nothing is known about the post release mortality rate of leatherback turtles, but bycatch was estimated as 3529 leatherback turtles (95% CI: 2463–4889). Regarding bottom trawling, the whole fleet was estimated to case annually the death of 151 green turtles (95% CI: 67–191), 95 loggerhead turtles (95% CI:33–108) and 42 olive ridley turtles (95% CI: 23–87). Finally, purse seiners were estimated to cause negligible mortality to loggerhead and leatherback turtles, the only species bycaught by that fleet. Our study provides comprehensive and precise baseline data on sea turtle bycatch within the three main industrial fishing activities, crucial for understanding the demographic impacts on sea turtle populations. Moreover, it underscores the CCLME as a significant hotspot for marine megafauna bycatch.